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Article

Transformation of Phytoplankton Communities in the High Arctic: Ecological Properties of Species

by
Larisa Pautova
,
Vladimir Silkin
*,
Marina Kravchishina
and
Alexey Klyuvitkin
Shirshov Institute of Oceanology, Russian Academy of Sciences, 36 Nakhimovsky Pr, 117977 Moscow, Russia
*
Author to whom correspondence should be addressed.
Diversity 2025, 17(10), 703; https://doi.org/10.3390/d17100703
Submission received: 11 September 2025 / Revised: 30 September 2025 / Accepted: 6 October 2025 / Published: 8 October 2025
(This article belongs to the Section Marine Diversity)
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Abstract

During the 84th cruise of the R/V Akademik Mstislav Keldysh in August 2021, patterns of phytoplankton composition transformation were revealed along a northward gradient. The study involved three transects in the Fram Strait and adjacent Arctic waters: a southern transect (from the Barents Sea shelf to the Greenland shelf), a middle transect across the Fram Strait, and a northern transect along the ice edge. Ten species of diatoms and eleven of dinoflagellates were identified, and their ecological preferences were characterized by determining the minimum, maximum, mean, and median values for abundance, biomass, depth of the biomass maximum, salinity, temperature, and the concentrations and ratios of nitrogen, phosphorus, and silicon. Significant gradients in temperature, salinity, silicon, and nitrogen concentrations were recorded along the south–north direction in the study area. The phytoplankton community responds to these changing factors through restructuring. Dinoflagellates predominantly dominate the southern and middle transects, whereas large diatoms make a substantial contribution to the phytoplankton biomass in the northern transect. Diatom biomass is determined by nitrogen concentration. The dependence of dinoflagellate biomass on that of small flagellates confirms the importance of mixotrophic nutrition. A hypothesis is proposed that the most probable criterion for the selective selection of diatoms northward is the half-saturation constant for nitrogen uptake, while for dinoflagellates, it is temperature.

1. Introduction

Climate change is most pronounced in the Arctic, primarily driven by the rapid reduction of sea ice cover [1,2,3]. The expansion of open-water areas reduces albedo, leading to further warming of surface waters [4]. Furthermore, the structure of the remaining ice is changing, with multi-year ice being replaced by first-year ice [5]. Projections suggest that the entire Arctic Ocean (AO) will be largely ice-free in summer within the coming decades [6,7]. A key hypothesis explaining these profound climatic shifts is the increased advection of warm and saline Atlantic Waters (AW) [8,9,10]. This process alters the vertical structure of water masses, ultimately restructuring existing marine ecosystems. In the Eurasian Arctic, this phenomenon is termed “atlantification” [8,9,11,12,13].
These changes affect both ice-associated and pelagic ecosystems. Shifts in ice age and physical structure lead to alterations in ice algal phytoplankton composition [14,15,16]. In the pelagic realm, boreal species are expanding northward, and the phenology and intensity of phytoplankton blooms are being modified [17,18,19,20,21,22,23,24]. These cascading effects impact the entire trophic web, from bacteria to fish [25,26,27,28,29,30]. Decreasing ice thickness enhances under-ice irradiance [31,32], which could potentially boost the productivity of Arctic pelagic ecosystems [33,34,35,36,37]. However, intense growth can also lead to more rapid nutrient depletion, potentially resulting in a significant decrease in overall ecosystem productivity [38].
The primary gateway for warm, saline Atlantic Water into the AO is the Fram Strait. Its eastern part is dominated by the northward-flowing West Spitsbergen Current (WSC) [39]. In contrast, the western part of the strait transports cold, freshened Arctic waters southward via the East Greenland Current. Both currents exhibit significant temporal and spatial variability [40,41,42].
Northward advection is accompanied by a decrease in a crucial ecological factor: temperature. The formation of community structures in response to changing temperatures involves complex processes that cannot be predicted solely from the physiological responses of individual species. An ecosystem’s response to warming involves the entire community, of which a species is only one part, and it cannot be considered in isolation [43,44,45]. Secondly, an individual species’ response to increasing temperature is contingent upon other environmental factors [46]. For autotrophic organisms, irradiance is paramount. As the ice edge retreats northward, key factors such as irradiance and photoperiod change [47]. Decreasing irradiance lowers the optimal growth temperature [48], which is also affected by nutrient limitation [49,50]. Consequently, studying a species’ response in monoculture (e.g., temperature growth curves) is insufficient for understanding the processes occurring during ocean warming. Obtaining such curves for all phytoplankton species is also impractical. Therefore, field observation data from the Arctic hold independent value. Logistical challenges often prevent the acquisition of long-term data series, making it difficult to disentangle climatic trends from interannual variability.
Another critical consideration is that phytoplankton community formation in the high Arctic occurs at temperatures far below the optimal growth temperatures for most species [45,51].
The aforementioned factors introduce significant uncertainty into our understanding of high Arctic phytoplankton community. In nature, species exist within a complex mosaic of physiological responses and ecosystem shifts, which must be accounted for when studying adaptation to changing environmental conditions [45,52]. Therefore, determining the ecological characteristics of the species that underpin the functioning of the high Arctic ecosystem is crucial. This information can serve as a basis for predicting the most probable ecosystem structure under a warming Arctic scenario.
This study aims to identify the ecological characteristics of the main phytoplankton species along the northward pathway of the AW through the Fram Strait. We simultaneously test the following hypothesis: adaptation to decreasing summer water temperatures in high latitudes occurs through the formation of a community dominated by large diatoms and mixotrophic dinoflagellates. To achieve this goal, we addressed the following objectives during the R/V Akademik Mstislav Keldysh cruise in the summer of 2021: (1) analyze the phytoplankton community structure, (2) assess the abundance and biomass of dominant species, and (3) determine their key ecological characteristics.

2. Materials and Methods

2.1. Field Studies

Seawater sampling for nutrient and phytoplankton analysis was conducted in the first half of August 2021 during the 84th cruise of the R/V Akademik Mstislav Keldysh along three transects: southern, middle, and northern (Figure 1, Table S1). The southern transect crossed the North Atlantic from the Barents Sea shelf in the east to the Greenland shelf in the west, around 74–76° N (Stations 7045–7052, Figure 1). The middle transect began at the southern tip of Spitsbergen, traversed the Fram Strait from the western coast of Spitsbergen to the ice edge in the northern part of the strait (Stations 7063–7068 and 7084–7089). The northern transect ran from the shelf edge of the Yermak Plateau in the south-western along the ice edge to the north-eastern in the periphery of the Nansen Basin and further across the shelf of Northern Svalbard (Stations 7083–7069).

2.2. Species Identification

Species were identified based on morphological characteristics using the taxonomic guides in [53,54] and the WoRMS database (https://www.marinespecies.org/, accessed on August 2025). Identification and cell counting were performed using an Ergaval light microscope (Karl Zeiss, Jena, Germany) at magnifications of 16 × 20 and 16 × 40.
Cells with linear dimensions less than 20 µm were counted in a Najotte chamber (0.05 mL). Larger cells were counted in a Naumann chamber (1 mL). Unidentified species measuring 4–10 µm were classified as small flagellates. Cells smaller than 2 µm were excluded from the total phytoplankton biomass assessment. Cell biovolume and biomass were calculated according to [55]. For ecological characterization, we selected species with at least 5 occurrences. The dominant species had the highest biomass at a given depth; the subdominant species was second after the dominant species.

2.3. Nutrient Analysis

The concentrations of phosphate phosphorus (P-PO4), dissolved silicon (Si), nitrate (N-NO3), nitrite (N-NO2), and ammonium (N-NH4) were determined using a Technicon II segmented flow autoanalyzer or by standard colorimetric methods [56,57]. Dissolved inorganic nitrogen (DIN) was calculated as the sum of nitrate, nitrite, and ammonium.
The euphotic depth was defined as the depth where underwater photosyntheti-cally active radiation (PAR, the visible light in the spectral range at 400–700 nm) decreased to 1% and 0.1% of its surface value. A LiCOR (Lincoln, NE, USA) complex based on photodiode sensors PHAR LI-192 LiCOR (measuring total irradiance in the range of 400–700 nm) were used to measure underwater PAR.

2.4. Ice Cover

Ice cover imagery for the period 16–18 August 2021 was provided by the State Scientific Center “Arctic and Antarctic Research Institute” (electronic atlas: https://data.aari.ru//odata/_d0015.php, accessed on 18 August 2021).

2.5. Statistical Analysis

A significance level of α = 0.05 was used for all statistical tests. Student’s t-test was employed to determine significant differences between sample groups. For paired data, Pearson’s rank correlation coefficients between hydrological parameters and biological variables were calculated. All statistical analyses were performed using PAST version 4.17 software (https://www.nhm.uio.no/english/research/resources/past accessed on 15 July 2024).

3. Results

3.1. Hydrography

We adopted the water mass terminology proposed by [58], with boundaries defined according to [59,60]. We distinguished Atlantic Water (AW) and Polar Surface Water (PSW), using salinity as the primary descriptor (Table 1). Waters with salinity above 34.75 were classified as AW, and those below 34.0 as PSW. The intermediate salinity range (34.0 < S < 34.75) was defined as modified AW (mAW). Within PSW, we identified meltwater-influenced PSW (mPSW) with temperatures below 0 °C.
The Si concentration in the euphotic layer was more than two times lower in AW compared to PSW (Table 1 and Table S2), while the DIN concentration was three times higher in AW.

3.2. Phytoplankton Community Composition

The total phytoplankton biomass averaged below 100 mg m−3 (Table 2). Dinoflagellates were the main contributors to the total biomass. Diatom biomass was, on average, more than two times lower than dinoflagellate biomass; however, it was relatively high at stations on the northern transect, reaching a maximum of 617.1 mg m−3 at 34 m depth at Station 7082 (Table S3). At this station, diatoms dominated the phytoplankton community. Only three diatom and two dinoflagellate species exhibited intensive growth with biomass exceeding 100 mg m−3. The maximum dinoflagellate biomass was observed on the southern transect (Table 3).
The haptophyte Phaeocystis pouchetii (Hariot) Lagerheim, 1896 reached high abundances and biomass on the northern transect; at Station 7081 (35 m depth), its contribution to the total phytoplankton biomass exceeded 50% (Table 2 and Table 3 and Table S2).
Ciliates grew intensively on the northern transect, achieving their highest biomass at Station 7075, where they contributed over 99% to the total phytoplankton biomass (Table 3 and Table S3).
Coccolithophore growth was minimal, with a slight presence observed only on the southern transect; they were virtually absent on the northern transect (Table 2 and Table S3). The contribution of small flagellates to the total phytoplankton biomass was minor, averaging just over 10%.

3.3. Diatoms

Among the diatoms that showed significant biomass, we identified 10 species (Tables S4 and S6). Only three species exceeded a biomass of 100 mg m−3 (Table 3), all located on the northern transect (Figure 1). The maximum diatom biomass was recorded at Station 7082 (34 m depth), driven by the large diatom Rhizosolenia styliformis T. Brightwell, 1858. Two other diatoms, Thalassiosira rotula Meunier, 1910 and Rhizosolenia hebetata f. hebetata J.W. Bailey, 1856, grew less intensively, reaching their maximum biomass at greater depths (34–35 m) at Stations 7070 and 7081, respectively. Two additional diatom species, Chaetoceros borealis Bailey, 1854 and Eucampia groenlandica Cleve, 1896, achieved maximum biomass values exceeding 50 mg m−3. The maximum biomass of the remaining species was lower.
All ten diatom species were found throughout the euphotic zone, but their median depths of occurrence differed significantly (Table 4 and Table S3). The median depth of maximum biomass was negatively correlated with cell volume (CC = −0.71, p = 0.01) (Figure 2), indicating that larger cells tended to dominate at shallower depths.
Regarding salinity, only one diatom species, Porosira glacialis (Grunow) Jörgensen, 1905, was predominantly associated with PSW (Figure 3, Table S4). The diatoms Eucampia groenlandica, Thalassiosira rotula, Rhizosolenia semispina (Hensen) Gran, 1908, and Rhizosolenia hebetata f. hebetata were most common in mAW, while the remaining species dominated in AW.
Diatoms grew across a wide temperature range, from negative values characteristic of meltwaters (mPSW) to temperatures above 8 °C found only in AW (Table 4 and Table S4). The diatoms Eucampia groenlandica, Rhizosolenia semispina, R. hebetata f. hebetata, and Porosira glacialis were characteristic of waters with temperatures up to 3 °C; Thalassiosira rotula, Chaetoceros borealis, and R. styliformis predominantly grew at temperatures between 3 and 5 °C (Figure 3). The remaining species were more frequently found at temperatures above 5 °C.
The identified diatoms grew at relatively high phosphorus concentrations (>0.1 µM L−1). They predominantly occurred at Si concentrations between 1 and 2 µM (Figure 4, Tables S2 and S4), with the exception of Porosira glacialis. Most species were found at DIN concentrations between 1 and 3 µM L−1; P. glacialis, Rhizosolenia semispina, and R. hebetata f. hebetata were associated with lower concentrations of this nutrient. Thalassiosira rotula was found at higher Si concentrations than T. gravida Cleve, 1896 (1.77 µM L−1 and 1.23 µM L−1, respectively; p = 0.028).
In PSW on the northern transect, diatom biomass increased with increasing DIN concentration (CC = 0.44, p = 0.006) (Figure 5).

3.4. Dinoflagellates

Among the dinoflagellates that showed significant growth, we identified 11 species (Table S5 and S7). The genus Protoperidinium was the most diverse (6 species), followed by Tripos (3 species); the genus Gyrodinium and Prorocentrum were represented by one species each.
Dinoflagellates of the genus Tripos and Protoperidinium depressum (Bailey, 1854) Balech, 1974 were found exclusively in AW (Figure 6). P. islandicum (Paulsen, 1904) Balech, 1973 was characteristic of PSW; the remaining dinoflagellate species were most frequently associated with mAW (Table S5). Dinoflagellates of the genus Tripos, P. depressum, and P. pyriforme subsp. breve (Paulsen) Balech, 1988 are typical for temperatures above 5 °C and can be classified as warm-water species. Only two species, P. islandicum and P. granii (Ostenfeld) Balech, 1974, were noted at temperatures below 2 °C and are considered cold-water species. The remaining species were successful at temperatures between 2 and 5 °C and are categorized as transitional species.
All dinoflagellates were observed at Si concentrations above 1 µM L−1 (Figure 7, Table S5). Protoperidinium depressum, P. islandicum, P. brevipes (Paulsen, 1908) Balech, 1974, and Gyrodinium lacryma (Meunier) Kofoid & Swezy, 1921 were associated with nitrogen-limited environments (DIN < 1 µM L−1); all others, except P. pyriforme subsp. breve, were found at DIN concentrations above 2 µM L−1.
Excluding representatives of the genus Tripos, the remaining species could be separated into cold-water and warm-water groups based on t-test comparisons of their mean occurrence temperatures (Figure 6). Protoperidinium granii, P. islandicum, Gyrodinium lacryma, and P. pellucidum Bergh, 1882 were classified as cold-water species. For the remaining species, higher environmental temperature was more characteristic.
Dinoflagellate biomass showed a significant positive correlation with the biomass of small flagellates (CC = 0.47, p = 10−5) (Figure 8). In contrast to diatoms, the depth of the biomass maximum for the selected dinoflagellate species was independent of cell volume (p = 0.69).

4. Discussion

4.1. Water Masses

Our data illustrate in WSC the transition from AW to mAW. Strong meridional gradients in temperature, salinity, and concentrations of nitrogen and silicon were observed. The euphotic layer of the AW on the southern transect can be considered silicon-limited, whereas the northern transect was typically nitrogen-limited. This shift in the primary limiting nutrient from silicon to nitrogen along the AW to PSW gradient is a key factor determining phytoplankton composition in the Arctic [59,61].

4.2. Phytoplankton Community

Our results provide ecological characteristics for individual phytoplankton species in the Arctic region, indicating the environmental conditions where these species are most likely to be found. The ability to grow intensively at low temperatures is linked to both physiological adaptations and a superior competitive ability for resources. Since the mechanisms of primary photosynthetic reactions (light harvesting and energy transfer) are largely temperature-independent [62], the response to decreasing temperature likely involves the cells’ ability to efficiently redistribute growth-limiting resources (nitrogen, silicon for diatoms) or secure food for mixotrophic growth (dinoflagellates). Furthermore, genetic adaptations, such as producing proteins that regulate membrane conductivity at sub-zero temperatures, determine survival boundaries [59]. These mechanisms primarily underpin the biogeographical distribution of species from the equator to the pole [63,64].
In summer, dinoflagellates dominated the phytoplankton on the southern transect. Moving northward, the relative importance of dinoflagellates decreased while that of diatoms increased. Recent studies using trophic markers have shown that zooplankton diet in the Arctic part of the Fram Strait is primarily based on diatoms, whereas dinoflagellates are more important in the subarctic North Atlantic part [65].

4.3. Diatoms

For Arctic diatoms near Svalbard, three biogeographic distribution types have been identified: Arctic, Arctic-temperate, and cosmopolitan [66]. Our results, based on salinity ranges, show that diatom communities near the ice edge are primarily represented by Atlantic species, indicating that advection plays a crucial role in structuring these communities, similar to processes observed under ice [67].
All identified diatom species are of Atlantic origin, except for Porosira glacialis, which is typical first-year ice species [68]. Diatoms were recorded across a wide temperature range from <0 °C to >8 °C (Table 4 and Table S4). We classified Eucampia groenlandica, Thalassiosira rotula, Rhizosolenia semispina, and R. hebetata f. hebetata as cold-water species (most frequent at T ≤ 3 °C). Chaetoceros borealis and R. styliformis were considered intermediate species (3–5 °C). Species more frequently observed at temperatures above 5 °C were classified as warm-water. This division is operational and based on a single year’s data. For instance, T. gravida and T. rotula bloom intensively on the northern Barents Sea shelf and are often considered cold-water species associated with first-year ice [69]. T. gravida is sometimes described as a polar species and T. rotula as temperate [45], while other studies classify T. gravida as Arctic-temperate [70]. Our research shows that T. gravida was more associated with warmer AW, while T. rotula was more frequent in mAW (Figure 3). This suggests that factors other than, or in interaction with, temperature determine their distribution, highlighting the complex interplay between species and community dynamics [45].
Most diatoms identified grew at Si concentrations of 1–2 µM L−1, a range where this nutrient concentration gradient strongly influences diatom growth [71,72,73]. The diatoms were predominantly large-celled species with volumes exceeding 3000 µm3 (Table S6). Given that the proportion of silica per unit cell volume decreases with increasing diatom size [59], the dominance of large diatoms under these conditions is ecologically coherent.
Three large diatom species (Porosira glacialis, Rhizosolenia semispina, and R. hebetata f. hebetata) were typically found at DIN concentrations below 1 µM L−1, i.e., in the zone of nitrogen-limited growth [74,75]. The remaining species occurred in the transition range from limitation to saturation (1–3 µM L−1) [76]. This supports the hypothesis that large diatom growth can be limited by nitrogen concentration. The positive correlation between diatom biomass and DIN concentration in the PSW of the northern transect (Figure 5) further supports this hypothesis. Given the differential responses of diatom species to nitrogen concentration, this factor becomes a key regulator of phytoplankton composition in summer when DIN concentration is minimal [77,78].
The negative correlation between the depth of maximum biomass and cell volume suggests that species with smaller cell volumes became relatively more important with depth. Since the specific light absorption coefficient decreases with increasing cell volume [79], larger cells are less competitive in lower light conditions. Thus, light availability also acts as a regulator of phytoplankton vertical composition.

4.4. Dinoflagellates

Our study showed that dinoflagellates dominated at most stations on the southern transect. On the middle transect, diatoms and coccolithophores were also among the dominants alongside dinoflagellates. On the northern transect, dinoflagellates and large diatoms were the main contributors to biomass. This suggests that in summer, the phytoplankton in the studied region was in the second or third stage of seasonal succession [80]. In open water, the spring bloom (March–April) is typically dominated by small diatoms (e.g., Chaetoceros, Fragilariopsis, Thalassiosira) [81,82,83], often followed by under-ice blooms of pennate diatoms [35]. Subsequent nutrient depletion leads to a decline in biomass and a shift towards flagellate dominance [82]. The strong correlation between dinoflagellate biomass and the biomass of the small flagellates (Figure 8) suggests phagotrophic grazing is a key mechanism. This underscores the need to consider these dinoflagellates as mixoplankton to understand their dynamics [84,85]. These dinoflagellates possess chloroplasts [86], allowing them to photosynthesize during the summer period of near-continuous illumination. The lack of a relationship between the depth of the biomass maximum and cell volume highlights the fundamental importance of the heterotrophic component. Thus, both autotrophic and phagotrophic processes occur within the same cell. The high biomass achieved by dinoflagellates indicates that this strategy is highly effective under subpolar summer conditions. Mixotrophy is widespread in Arctic seas [87] and must be considered when studying dinoflagellate responses to temperature change.
In our study, Protoperidinium islandicum was primarily found in PSW, though it also occurred at salinities characteristic of AW. All other species were more associated with AW, indicating that most studied dinoflagellates are of Atlantic origin.
Dinoflagellates of the genus Tripos and Protoperidinium depressum appeared sensitive to temperature changes. Species of Tripos are known to be expanding northward [88], but our data suggest that below 5 °C, they are outcompeted for resources by other dinoflagellates. This fact indicates that temperature is a key regulator of both trophic interactions [89] and global biogeographical patterns [90]. Although P. depressum existed over a wide temperature range (Table S5), it was most frequent in warm AW. We classified the remaining species into cold-water and warm-water categories pragmatically; for example, P. granii, P. islandicum, Gyrodinium lacryma, and P. pellucidum are most likely to be found at lower temperatures than the others.
All dinoflagellates were most often found at Si concentrations above 1 µM L−1, beyond the range of acute silicon limitation for diatoms, suggesting potential competition between diatoms and dinoflagellates for dissolved nitrogen.
The dinoflagellates Protoperidinium depressum, P. islandicum, P. brevipes, and Gyrodinium lacryma were associated with low DIN concentrations (<1 µM L−1), implying they acquire nitrogen primarily through phagotrophy (consuming other phytoplankton cells or bacteria). Other dinoflagellates (except P. pyriforme subsp. breve) were found at higher DIN concentrations (>2 µM L−1), suggesting autotrophy plays a more significant role in their biomass formation.
From a biogeographical perspective, all considered species are cosmopolitans [91]. The warm/cold-water classification is operational and reflects the species’ ecological response to temperature rather than strict biogeography. Some species are found at low temperatures not because they are cold-water endemics, but because they possess traits that enhance survival under those conditions (e.g., proteins for membrane function at low temperatures, low temperature optimum, colony formation, spore formation in diatoms [59]). For warm-water species in Si-limited AW, weakly silicified species gain an advantage. For mixotrophic dinoflagellates, the nature and size of prey are likely fundamental to their distribution [92], as is their ability to swim actively [93,94].

4.5. Main Patterns and Trends

A pronounced south-to-north gradient was observed for temperature, salinity, and the concentrations of silicon and nitrogen, indicating a multifactorial shift in environmental conditions. The phytoplankton community responds to these changing factors through restructuring, affecting both the diatom and dinoflagellate components. Many species dominant in the south disappear in the north of the studied area.
The primary trends associated with the northward direction are decreasing temperature and salinity, decreasing nitrogen concentration, and increasing silicon concentration. Consequently, the environmental filter [95] selects for species based on these prevailing trends. Identifying the principal driving factors is not a straightforward task. The temperature hypothesis is frequently considered [29,46,52,90]. For autotrophic species, such as diatoms, decreasing temperature selects for species with a relatively low optimal temperature and, consequently, a lower maximum specific growth rate. Therefore, the maximum specific growth rate observed under high nutrient concentrations is not the primary determinant for selective selection.
The second factor regulating species selection is nitrogen concentration. Diatoms in the northern transect are represented by large-celled species. Their biomass is determined by the ambient nitrogen concentration. High diatom biomasses are recorded at the interface between nitrogen-rich Atlantic Water (AW) and silicon-rich Polar Surface Water (PSW), provided this occurs within the euphotic zone. As shown in Figure 4, northern diatom species grow at nitrogen concentrations below 1 µM L−1. Consequently, according to competition theory [96], they possess a low half-saturation constant for nitrogen uptake. Thus, for diatoms, the most probable criterion for selective selection is the half-saturation constant for nitrogen uptake.
For dinoflagellates, the key trait is their ability to simultaneously derive energy from both photosynthesis and phagotrophy [84,85]. This mixotrophic strategy is flexible and regulated by environmental conditions. The dependence of dinoflagellate biomass on the biomass of small flagellates supports the concept that the mixoplankton paradigm should underpin the theory of plankton community transformations in the high Arctic. Here, temperature may act as a factor regulating prey quality or controlling the redistribution of energy between autotrophic and heterotrophic processes. Since nitrogen concentration was not a key factor distinguishing “northern” and “southern” dinoflagellate species, it does not play a significant role in their selective selection.
A distinctive feature of phytoplankton distribution in the north is its vertical segregation: dinoflagellates dominate the upper water layers, while large diatoms dominate the lower layers within the euphotic zone. While one perspective holds that mixotrophy prevails in tropical oligotrophic regions and strict phagotrophy is characteristic of high latitudes regions [97], the dominance of dinoflagellates in the upper layers under relatively high irradiance suggests that mixotrophy also plays an important role in the high Arctic.

5. Conclusions

The ecological characteristics identified for phytoplankton species allow us to predict shifts in community composition under climate change scenarios. In summer, within the relatively warm Atlantic Waters, phytoplankton biomass is dominated by dinoflagellates of the genus Tripos and Protoperidinium depressum. Moving northward into the Fram Strait, these species are replaced by P. pyriforme subsp. breve, P. brevipes, and Prorocentrum cordatum. A further decrease in temperature and salinity leads to the dominance of P. pellucidum and Gyrodinium lacryma. The dinoflagellates P. granii and P. islandicum are most successful at the lowest temperatures.
The contribution of diatoms in warm Atlantic Waters is minor, with Stephanopyxis turris, Navicula planamembranacea, and Thalassiosira gravida being noted. Northward, dominance shifts to T. rotula, Chaetoceros borealis, and Rhizosolenia styliformis. A further temperature decreases below 3 °C favors Eucampia groenlandica, R. semispina, and R. hebetata f. hebetata. The large diatom Porosira glacialis grows in mPSW.
The most likely criteria for the selective selection of diatoms during northward movement is the half-saturation constant for nitrogen uptake, and for dinoflagellates, it is temperature.
Diatom biomass is controlled by ambient nitrogen concentration, while dinoflagellate biomass is linked to the biomass of small flagellates, indicating that both autotrophic and mixotrophic processes govern the productivity of high Arctic ecosystems. Given that the diatom component is represented by large-celled species, our stated hypothesis—that the adaptation of phytoplankton composition to decreasing summer temperatures in high latitudes occurs through the formation of a community dominated by large diatoms and mixotrophic dinoflagellates—is supported by findings from our study in August 2021.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/d17100703/s1, Table S1: Date, time, station locations, and sampling depths; Table S2: Salinity (PSU), temperature (T, °C), concentrations of phosphorus (P, µM), nitrogen (DIN, µM), silicon (Si, µM), and their ratios; Table S3: Full taxonomic composition of the phytoplankton community; Table S4: Abundance, biomass, and ecological parameters (depth of maximal biomass (h), salinity (Sal), temperature (T), silicon (Si), phosphorus (P), nitrogen (DIN) concentrations) for dominant diatom species. Color coding: salinity (blue—PSW, yellow—mAW, red—AW); temperature (blue—cold-water, yellow—transitional, red—warm-water); Si (blue—Si-limiting, red—not limiting); DIN (blue—N-limiting, yellow—transitional, red—not limiting); Si:N ratio (blue—Si-limiting, yellow—transitional, red—N-limiting); Table S5: Abundance, biomass, and ecological parameters for dominant dinoflagellate species. Color coding: salinity (blue—PSW, yellow—mAW, red—AW); temperature (blue—cold-water, red—warm-water); DIN (blue—N-limiting, yellow—transitional, red—not limiting); Table S6: Average cell volume of diatoms; Table S7: Average cell volume of dinoflagellates.

Author Contributions

Conceptualization, L.P. and V.S.; Data curation, A.K.; Funding acquisition, M.K.; Investigation, L.P.; Methodology, V.S.; Project administration, M.K.; Resources, M.K.; Visualization, A.K.; Writing—original draft, L.P. and V.S.; Writing—review and editing, L.P. and V.S. All authors have read and agreed to the published version of the manuscript.

Funding

Phytoplankton analyses were carried out within the state assignment of the Ministry of Science and Higher Education of the Russian Federation for IO RAS (theme No. FMWE-2024-0023). Studies of M.K. and A.K. were supported by the Russian Science Foundation (project No. 25-17-00334, https://rscf.ru/project/25-17-00334/ accessed on 30 August 2025).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study were included in the article and Supplementary Materials; further inquiries can be directed to the corresponding author.

Acknowledgments

Field studies were carried out using the equipment of the unique scientific facility “R/V Akademik Mastislav Keldysh” as part of the Center for the collective use of scientific equipment “Scientific fleet of IO RAS”, https://rv.ocean.ru/en/flot/abf/nis-akademik-mstislav-keldyish (accessed on 10 September 2025). We gratefully acknowledge the captain, crew, and scientific group of the 84th cruise of the R/V Akademik Mstislav Keldysh (August 2021) for their support in sample acquisition. We thank Alexander Schuka and Ivan Zamyatin for their assistance with CTD data processing and Elena Kudryavtseva for help in collecting phytoplankton samples.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Map of sampling stations in 2021 (current scheme based on [40]). 1—southern transect (pink); 2—middle transect (yellow); 3—northern transect (blue). Red arrows show West Spitsbergen Current (WSC) and its Western and Eastern Branches.
Figure 1. Map of sampling stations in 2021 (current scheme based on [40]). 1—southern transect (pink); 2—middle transect (yellow); 3—northern transect (blue). Red arrows show West Spitsbergen Current (WSC) and its Western and Eastern Branches.
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Figure 2. The dependence of the depth of occurrence of the maximum biomass of diatom species (H, m) on their cell volume. R2 = 0.50, p = 0.01, n = 11, 95% ellipses.
Figure 2. The dependence of the depth of occurrence of the maximum biomass of diatom species (H, m) on their cell volume. R2 = 0.50, p = 0.01, n = 11, 95% ellipses.
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Figure 3. Distribution of the identified diatom species in salinity–temperature coordinates. P. glacialis—Porosira glacialis; R. semispina—Rhizosolenia semispina; R. h. f. hebetata—Rhizosolenia hebetata f. hebetata; E. groenlandica—Eucampia groenlandica; T. rotula—Thalassiosira rotula; R. styliformis—Rhizosolenia styliformis; C. borealis—Chaetoceros borealis; T. gravida—Thalassiosira gravida; N. planamembr—Navicula (=Ephemera) planamembranacea) (Hendey) Paddock 1988; E. turris—Eupyxidicula turris (Grev. & Arn.) S. Blanco & C.E. Wetzel, 2016.
Figure 3. Distribution of the identified diatom species in salinity–temperature coordinates. P. glacialis—Porosira glacialis; R. semispina—Rhizosolenia semispina; R. h. f. hebetata—Rhizosolenia hebetata f. hebetata; E. groenlandica—Eucampia groenlandica; T. rotula—Thalassiosira rotula; R. styliformis—Rhizosolenia styliformis; C. borealis—Chaetoceros borealis; T. gravida—Thalassiosira gravida; N. planamembr—Navicula (=Ephemera) planamembranacea) (Hendey) Paddock 1988; E. turris—Eupyxidicula turris (Grev. & Arn.) S. Blanco & C.E. Wetzel, 2016.
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Figure 4. Distribution of the identified diatom species in silicon-nitrogen (DIN) coordinates. P. glacialis—Porosira glacialis; R. semispina—Rhizosolenia semispina; R. h. f. hebetata—Rhizosolenia hebetata f. hebetata; E. groenlandica—Eucampia groenlandica; T. rotula—Thalassiosira rotula; R. styliformis—Rhizosolenia styliformis; C. borealis—Chaetoceros borealis; T. gravida—Thalassiosira gravida; N. planamembr—Navicula (=Ephemera) planamembranacea); E. turris—Eupyxidicula turris.
Figure 4. Distribution of the identified diatom species in silicon-nitrogen (DIN) coordinates. P. glacialis—Porosira glacialis; R. semispina—Rhizosolenia semispina; R. h. f. hebetata—Rhizosolenia hebetata f. hebetata; E. groenlandica—Eucampia groenlandica; T. rotula—Thalassiosira rotula; R. styliformis—Rhizosolenia styliformis; C. borealis—Chaetoceros borealis; T. gravida—Thalassiosira gravida; N. planamembr—Navicula (=Ephemera) planamembranacea); E. turris—Eupyxidicula turris.
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Figure 5. Relationship between diatom biomass and DIN (µM L−1) concentration in PSW on the northern transect. R2 = 0.20, p = 0.006, n = 37, 95% ellipses.
Figure 5. Relationship between diatom biomass and DIN (µM L−1) concentration in PSW on the northern transect. R2 = 0.20, p = 0.006, n = 37, 95% ellipses.
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Figure 6. Distribution of the identified dinoflagellate species in salinity–temperature coordinates. T. —Tripos; P. cordatum—Prorocentrum cordatum; G. lacryma—Gyrodinium lacryma; P. breve—Protoperidinium pyriforme subsp. breve (Paulsen) Balech, 1988. The remaining species represent the genus Protoperidinium.
Figure 6. Distribution of the identified dinoflagellate species in salinity–temperature coordinates. T. —Tripos; P. cordatum—Prorocentrum cordatum; G. lacryma—Gyrodinium lacryma; P. breve—Protoperidinium pyriforme subsp. breve (Paulsen) Balech, 1988. The remaining species represent the genus Protoperidinium.
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Figure 7. Distribution of the identified dinoflagellate species in silicon-nitrogen (DIN) coordinates. T.—Tripos; P. cordatum—Prorocentrum cordatum; G. lacryma—Gyrodinium lacryma; P. breve—Protoperidinium pyriforme subsp. breve (Paulsen) Balech, 1988. The remaining species represent the genus Protoperidinium.
Figure 7. Distribution of the identified dinoflagellate species in silicon-nitrogen (DIN) coordinates. T.—Tripos; P. cordatum—Prorocentrum cordatum; G. lacryma—Gyrodinium lacryma; P. breve—Protoperidinium pyriforme subsp. breve (Paulsen) Balech, 1988. The remaining species represent the genus Protoperidinium.
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Figure 8. Relationship between the logarithm of dinoflagellate biomass and the logarithm of biomass of small flagellate biomass. R2 = 0.22, p = 10−5, n = 81, 95% ellipses.
Figure 8. Relationship between the logarithm of dinoflagellate biomass and the logarithm of biomass of small flagellate biomass. R2 = 0.22, p = 10−5, n = 81, 95% ellipses.
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Table 1. Characteristics (median values) of potential temperature (θ, °C), salinity (PSU), dissolved silicon (Si, µM L−1), and dissolved inorganic nitrogen (DIN, µM L−1) in the core of defined water masses in the study area. The p-value indicates the significance of the difference between AW and PSW (Mann–Whitney U test).
Table 1. Characteristics (median values) of potential temperature (θ, °C), salinity (PSU), dissolved silicon (Si, µM L−1), and dissolved inorganic nitrogen (DIN, µM L−1) in the core of defined water masses in the study area. The p-value indicates the significance of the difference between AW and PSW (Mann–Whitney U test).
Water MassPotential TemperatureSalinitySiDIN
AWθ > 1.54434.75 < S < 35.030.691.58
mAWθ < 4.134.0 < S < 34.75--
PSW−1.659 < θ < 3.031.30 < S < 34.01.830.52
mPSW−1.6 < θ < 030.85 < S < 31.28--
p (AW vs. PSW) 7.3 × 10−99 × 10−6
Table 2. Statistical indicators of biomass (mg m−3) for the main taxonomic and size groups of phytoplankton (Diatoms, Dinoflagellates, Coccolithophores, Haptophyta, Small flagellates) and their percentage contribution to the total biomass.
Table 2. Statistical indicators of biomass (mg m−3) for the main taxonomic and size groups of phytoplankton (Diatoms, Dinoflagellates, Coccolithophores, Haptophyta, Small flagellates) and their percentage contribution to the total biomass.
GroupBiomass (mg m−3)Contribution (%)
MaxMeanMedianMaxMeanMedian
Diatoms617.119.91.6596.021.01.0
Dinoflagellates366.047.510.698.050.049.0
Coccolithophores47.12.10.0192.09.00.01
Haptophyta (Phaeocystis)206.22.70.0180.00.30.01
Small flagellates56.93.92.377.013.07.0
Total Phytoplankton670.866.825.6
n (number of samples)205205205205205205
Table 3. Dominant species with cell volume (Vcell, µm3), maximum abundance (cells L−1), maximum biomass (mg m−3), and its maximum contribution to total phytoplankton biomass.
Table 3. Dominant species with cell volume (Vcell, µm3), maximum abundance (cells L−1), maximum biomass (mg m−3), and its maximum contribution to total phytoplankton biomass.
StationDepth (m)Dominant SpeciesAbundance
(Cells L−1)		Biomass
(mg m−3)		Vcell (µm3)Contribution (%)
Diatoms
708234Rhizosolenia styliformis2300535.3247,40089.3
708135Rhizosolenia hebetata
f. hebetata		14,500145.510,00039.0
707035Thalassiosira rotula36,400170.9470086.7
Ciliates
70754Mesodinium rubrum58,200324.7558099.3
Dinoflagellates
70691Gyrodinium lacryma380304.0795,80087.1
70456Protoperidinium depressum80114.51,437,00067.0
Haptophyta
708135Phaeocystis pouchetii727,300196.427052.2
Table 4. Statistical indicators of abundance, biomass, depth of occurrence, salinity, temperature, concentrations of phosphorus, nitrogen, silicon, and their ratios for the 10 diatom species (n = number of measurements).
Table 4. Statistical indicators of abundance, biomass, depth of occurrence, salinity, temperature, concentrations of phosphorus, nitrogen, silicon, and their ratios for the 10 diatom species (n = number of measurements).
ParameternMinMaxMeanMedian
Abundance (cells L−1)2825112,5001629112
Biomass (mg m−3)2820.04535.2813.292.48
Depth (m)2751.050.018.516.0
Salinity (PSU)27330.9735.0434.2234.65
Temperature (°C)273−1.638.823.784.38
Phosphorus (P, µM L−1)2520.131.980.360.30
Silicon (Si, µM L−1)2520.514.561.731.48
Nitrogen (DIN, µM L−1)2320.289.652.612.00
N:P ratio2420.8517.816.805.88
Si:N ratio2420.17.81.40.6
Si:P ratio2461.216.05.75.1
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Pautova, L.; Silkin, V.; Kravchishina, M.; Klyuvitkin, A. Transformation of Phytoplankton Communities in the High Arctic: Ecological Properties of Species. Diversity 2025, 17, 703. https://doi.org/10.3390/d17100703

AMA Style

Pautova L, Silkin V, Kravchishina M, Klyuvitkin A. Transformation of Phytoplankton Communities in the High Arctic: Ecological Properties of Species. Diversity. 2025; 17(10):703. https://doi.org/10.3390/d17100703

Chicago/Turabian Style

Pautova, Larisa, Vladimir Silkin, Marina Kravchishina, and Alexey Klyuvitkin. 2025. "Transformation of Phytoplankton Communities in the High Arctic: Ecological Properties of Species" Diversity 17, no. 10: 703. https://doi.org/10.3390/d17100703

APA Style

Pautova, L., Silkin, V., Kravchishina, M., & Klyuvitkin, A. (2025). Transformation of Phytoplankton Communities in the High Arctic: Ecological Properties of Species. Diversity, 17(10), 703. https://doi.org/10.3390/d17100703

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